This type of accident is known in the technical literature by the acronym CFIT, standing for “Controlled Flight Into Terrain”. Although it used to represent a major proportion of air disasters, CFIT-type accidents are now mostly avoided, thanks to terrain avoidance maneuvers performed by the air crews, urged by alerts and alarms originating from onboard automatic collision risk signaling systems, TAWS (Terrain Awareness & Alerting Systems), which include the GCAS system (Ground Collision Avoidance System) and the T2CAS system (Terrain & Traffic Collision Avoidance System), developed and marketed by Thales.
The instruction given to an air crew confronted with a risk of collision with the terrain is to engage an avoidance maneuver in accordance with a predefined avoidance procedure which corresponds to a pure vertical avoidance maneuver, called “Pull-Up”, consisting of a full-throttle climb preceded by a leveling of the wings if the aircraft was turning. This maneuver, called “standard avoidance maneuver” or “SVRMB”, (“Standard Vertical Recovery Maneuver Boundary”), is independent of the aircraft type and of its instantaneous climbing capabilities. However, for the instruction to be effective, the standard avoidance maneuver still needs to be undertaken in time.
Onboard equipment that automatically signals flight situations leading to risks of collision with the terrain, sufficiently in advance for an actual vertical avoidance maneuver to be effective, have been developed in recent years. Among such equipment, the TAWS systems are the most efficient because they use a function FLTA (Forward-Looking Terrain Avoidance) which looks, in front of the aircraft, along and below its path vertically and laterally, to see if there is a potential risk of collision with the terrain.
The principle of the TAWS systems is based on monitoring the penetration of the terrain into one or more protection volumes linked to the aircraft based on a modeling of the terrain being flown over. The reliefs of the region being flown over are listed in a digital map that can be accessed on the aircraft. The position of the aircraft in relation to the region being flown over is supplied by flight equipment such as: inertial unit, satellite positioning receiver, baro-altimeter, radio-altimeter or a combination of several of these sensors. The protection volumes linked to the aircraft are advantageously defined to contain a modeling of the standard vertical avoidance maneuver path engaged in the more or less short term from the path followed by the aircraft predicted from flight parameters delivered by the aircraft flight equipment, assuming that the aircraft maintains its ground speed vector or its path. There are normally two of the protection volumes linked to the aircraft, of graduated sizes, the most forward one being used to give an alert indicating to the crew of the aircraft that the path being followed must be modified in the short term to avoid the terrain, and the nearer one being used to give an alarm indicating to the crew of the aircraft that it must actually, as an emergency, undertake a vertical avoidance maneuver.
Most of the onboard TAWS systems deliver, in addition to audible alerts and alarms intended to attract the attention of the crew of an aircraft to the need to modify the path being followed in the more or less short term, a cartographic representation on screen of the reliefs appearing in front and to the sides of the aircraft, from a subdivision of the terrain being flown over into various strata distinguishing between reliefs that are not threatening because they are well below the flight altitude range of the aircraft, the reliefs to be taken into consideration because they fall within the flight altitude range of the aircraft and the reliefs that are very threatening because they are above the flight altitude range of the aircraft. This on-screen cartographic representation of the threatening reliefs gives a good overview of the surrounding situation in a normal flight but it is not detailed enough to enable a crew to devise a relief avoidance path in the case of an alert, and even more so of an alarm regarding a risk of collision with the ground.
For more details on the concepts involved in the TAWS systems, the reader can profitably refer to the US patents U.S. Pat. Nos. 5,488,563, 5,414,631, 5,638,282, 5,677,842, 6,088,654, 6,317,663, 6,480,120 and to the French patent applications FR 2.813.963, FR 2.842.594, FR 2.848661, FR 2.860.292, FR 2.864.270, FR 2.864.312, FR 2.867.851, FR 2.868.835.
However, in some situations a standard vertical avoidance maneuver is insufficient to avoid a collision with the terrain and a more sophisticated avoidance maneuver with change of heading, hereinafter called lateral avoidance maneuver must be envisaged. Typically, such situations are encountered when the aircraft is turning along major relief, or even when it is directed in a straight line towards reliefs, particularly higher reliefs, that cannot be crossed given its climb capability. Some operational TAWS systems, including those developed by Thales, signal these particular situations with a specific “Avoid Terrain” alarm. However, they give no indication as to the path of the avoidance maneuver to be followed in such situations.
There is therefore a need in the operational TAWS systems to accompany “Avoid Terrain” type alarms with one or more complementary indications to help the crew choose a lateral avoidance path that is safe, appropriate to the hazardous situation causing the generation of an “Avoid terrain” alarm. More generally, indications on the path of the appropriate avoidance maneuver or on the sector of the space in which this path must be contained would be very useful with this type of alarm.
Moreover, since the crew can sometimes be in situations where it is not fully aware of the imminence of the danger or where it delays the avoidance maneuver, it is also desirable to be able to go beyond complementary indications as to the lateral avoidance path to be followed to an automation of the process of following this lateral avoidance path with or without supply of these complementary indications.
It should be noted that there are terrain-following systems that are operational onboard certain combat airplanes which, by automatic maneuvers, keep the airplane at a constant height above the ground despite the terrain accidents and which, because of this, perform automatic terrain avoidance maneuvers if the airplane comes too close to the relief. However, these onboard terrain-following systems are designed on the basis of very frequent terrain-hugging maneuvers and are not suited to aircraft such as transport airplanes, civilian or military, helicopters, or even drones in certain uses, most of whose missions are to follow programmed paths, manually or automatically, by means of a flight management computer FMS or an automatic pilot PA, as far as possible avoiding any significant unplanned alteration of these paths, except to avoid a collision with the terrain. For such aircraft, terrain avoidance maneuvers must be triggered as infrequently as possible to reduce the fatigue forces on the cell and to respect the comfort of the passengers.
As described in the American patent U.S. Pat. No. 4,924,401, there is also a ground collision prevention system, onboard and operational on combat airplanes but of a type that is different from a TAWS system because it takes account only of the flight parameters of the aircraft, which automatically undertakes standard, purely vertical, terrain avoidance maneuvers.
There are also automatic pilots for aircraft which, on command from a terrain collision prevention device, undertake to automatically follow a standard, purely vertical, terrain avoidance maneuver, by overriding the manual flight controls, which can be overridden by the pilot as described in the American patent U.S. Pat. No. 6,675,076 or which is imposed in an authoritarian way as described in the European patent application EP 1.517.211.
Also known, from patent U.S. Pat. No. 5,892,462, is a TAWS system that uses a modeling of the behavior of the aircraft to create a protection volume linked to the aircraft and to plot avoidance paths that are practicable for the aircraft. The protection volume surrounds an extrapolation of the current path of the aircraft extended by a climb path prediction. Its penetration by the terrain is likened to a risk of collision with the terrain. In the case of an established risk of collision with the terrain, the system works back step by step along the current path extrapolation to determine the limit-point of the path of the aircraft as far as where a purely vertical standard avoidance maneuver can be applied with success. When the limit-point is passed or can no longer be determined, the system initiates a search for a practicable lateral avoidance path by an angular sweep of the terrain profiles, either side of the current path of the aircraft.
This TAWS system gives an alert when the limit-point for application of a standard vertical avoidance maneuver is passed and generates an alarm when it appears that it will no longer be possible to find a lateral avoidance path. It addresses the concerns of better assistance to a crew confronted with the need to execute a lateral avoidance maneuver because it is also capable of acting on the flight director to guide the pilot in following a lateral avoidance path or on the automatic pilot to execute a lateral avoidance path. This system, which appears not to be operational at the present time, presents the drawback of using a modeling of the dynamic behavior of the aircraft which is complex to perform and which raises significant certification problems with a view to operational implementation because the level of accuracy of this behavioral modeling affects the dependability of the TAWS system itself.